Antennas are used for audio systems of vehicles in order to receive radio waves from an external radio transmitting station. Pole antennas attached to vehicle bodies and on-glass antennas fixed to a window (typically, a rear window) of the vehicle are typical examples of such antennas.
Pole antennas show good performance at receiving waves, but they cause an increase in manufacturing cost and mounting processes, and show defects of easy contamination and other problems. Therefore, on-glass antennas have been widely adopted in recent vehicles.
Vehicle audio systems also receive waves reflected from natural/man-made obstacles such as mountains and buildings, as well as waves directly from the transmitting station. Regions where such reflected waves are dominant are frequently called multi-path regions. When a vehicle is running in such a multi-path region, strength of the reflected and direct waves included in the received waves fluctuate such that the strength of the total waves (sum of the reflected and direct waves) also fluctuates.
On the other hand, during processing of radio waves by on-glass antennas, wave signals received at the on-glass antennas are amplified by an amplifier by approximately 5–10 dB, and accordingly a tuner detects the amplified wave signals.
During the amplification of the on-glass antenna signals, noise due to the fluctuation of wave strengths is also amplified. Especially in such a multi-path region, the noise produced at the amplifier becomes great such that noise mixed in sounds also becomes great.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.